Portable devices exist that provide radio frequency (RF) receiver functionality including RF receiver functionality in the AM broadcast band (about 520 to 1710 KHz). These prior radio devices have used receive antennas to receive broadcast channels. In particular, prior AM radio receiver systems have used air loop antennas to receive AM broadcast channels. These AM air loop antennas have been widely used in desktop radio applications, miniature high fidelity systems, home theater systems, etc. The air loop antennas are typically located away from the radio circuitry itself to make the air loop antenna much less susceptible to noise sources commonly caused by the other electronics associated with the radio device. In addition, the orientation of the external air loop antenna is independent of the placement of the radio device.
The AM air loop antenna has a small inductance, and the common practice is to insert a transformer between the air loop antenna and the radio circuitry for impedance transformation and matching. The transformer acts to increase the inductance and reduce the capacitance as well as increase the magnetically coupled voltage signal seen by the radio device from the air loop antenna. Traditionally, the transformer is completely separated from the air loop antenna and is mounted on a printed circuit board (PCB) along with the radio receiver circuitry.
FIG. 1 (prior art) is a diagram for a traditional solution 100 for a radio device 120 having an air loop antenna 114 and an internal transformer 104. As depicted, a PCB 106 includes a radio integrated circuit (IC) 102 that is mounted to or positioned on the PCB 106. The radio IC 102 includes tuner circuitry for receiving and tuning broadcast channels, such as broadcast channels in the AM band (about 520 to 1710 KHz). In addition, a transformer 104 is coupled to the PCB 106. The antenna receive signal connections 110 from the radio IC 102 are coupled to transformer 104, and the antenna receive signal connections 108 from transformer are coupled to external antenna connection points 118A and 118B. The antenna connections 112 for the air loop antenna 114 then connect to the antenna connection points 118A and 118B. In operation, the performance of the air loop antenna 114 is improved by the transformer 104. It is noted that the air loop antenna 114 is configured and used for reception of broadcast channels and AM band broadcast channels in particular.
The traditional solution, however, has a number of disadvantages. The transformer takes up space on the PCB and increases the size required for the PCB thereby increasing the size of the radio device. Further, due to the close proximity of the transformer to the radio IC and other radio electronics on the PCB, the transformer tends to pick up noise thereby degrading radio performance. A typical solution to this interference problem is to heavily shield the transformer and/or place the transformer on the PCB at a further distance from the noise sources on the PCB. This solution, however, further causes increases in the size of the PCB which in turn causes increases in the size of the radio device. In addition, electromagnetic shielding in the AM frequency band can be challenging without a lot of BOM (build of materials) increases.